Caption: MODEL RELEASED. Catalytic hardening of filler. Catalyst being used to harden a filler. The filler (squeezed from the tube at left) needs to have the catalyst at centre (red) added to it to make it harden. The hardener contains free radicals that initiate the setting process, triggering a cascade of bond formation that turns the soft filler material (being prepared at right) into a hard, binding solid. Fillers are materials used to fill in gaps in masonry and brickwork, acting like a fast-setting cement. This is the P38 filler from Isopon. The hardener is benzoyl peroxide (BPO). See A500/683 for another example of this process.